In the field of public address systems, one of the most important problems is to freely control the directivity of sound. In particular, as noise pollution has recently become a social problem, demands have increased for a direction variable, or direction controlled, loudspeaker system. However, since the wavelength of a sound wave is extremely long as compared with light, it has been difficult to realize a loudspeaker system having super directivity like a spot-light while a wide directivity can be readily realized.
Hitherto, a horn loudspeaker has been mainly used to sharpen the directivity, but there is a drawback in that a gigantic horn is necessitated to sharpen the directivity to low frequencies such as the voice band.
On the other hand, a loudspeaker system utilizing the nonlinear interaction between finite amplitude ultrasonic waves by the nonlinearity of a medium (parametric loud-speaker) has recently drawn attention because it can give super directivity as compared with the conventional system (Japanese Laid-open Patent Publication No. 58-119293). However, chiefly for the following reasons, the parametric loudspeaker has not long been used in practice.
(1) Because of a low conversion efficiency, an extremely powerful ultrasonic wave is required to reproduce an audible sound of practically acceptable level, and when listeners are subjected directly to this powerful ultrasonic sound, harm such as hearing impairment will occur.
(2) Since a space which is called a parametric array is required to reproduce an audible sound from the ultrasonic wave, the loudspeaker system is increased in length and the space for installation is limited.
(3) Because of the low conversion efficiency, the use of a very bulky and expensive ultrasonic wave radiator is required in order to cover a large listening area.
(4) As is the case with the conventional loudspeaker, the directivity cannot be freely controlled.
In order to control the directivity of a loudspeaker, a loudspeaker having super directivity is necessary in the first place. This is because, if the super directivity is realized, any directional characteristic can be realized by combination therewith. Hitherto, as a loudspeaker having super directivity, a horn loudspeaker has been used chiefly. This is, as shown in FIG. 1, a version wherein an acoustic tube 2 having its cross-sectional area varying gradually, which is called a horn, is fitted frontwardly to a dynamic electroacoustic transducer 1 which is called a driver. However, the directional characteristic of the horn loudspeaker depends mainly on the shape of a horn side wall 3 and the length of the horn, and there is a problem in that an extremely long horn is necessary in order to have super directivity at a low frequency. It is to be noted that 3a represents a movable side wall.
On the other hand, the parametric loudspeaker, which is a sound reproducing system utilizing a nonlinear effect, is capable of realizing super directivity comparable to the conventional loudspeaker utilizing a linear phenomenon, even though it has a radiating surface area of a size equal to one tenth of that of the conventional loudspeaker. Hereinafter, the fundamental principle of the parametric loudspeaker will be described with reference to FIG. 2.
In FIG. 2, 4 represents a source of an audio signal to be reproduced, 5 represents a high frequency oscillator used in a carrier wave, 6 represents a modulator, 7 represents a power amplifier, and 8 represents an ultrasonic wave radiator. The audio signal source 4 and an output signal from the high frequency oscillator 5 for the carrier wave are inputted to the modulator 6. An output signal from the modulator is amplified by the power amplifier 7, inputted to the ultrasonic radiator 8, and radiated in the air in the form of an ultrasonic wave modulated by the audio signal.
Where a sound wave has a high amplitude and is considered having a finite amplitude, the original waveform is distorted by the nonlinearity of a medium (e.g. air) and numerous frequency components not included in the original waveform tend to be produced as it propagates. The parametric loudspeaker utilizes one of the nonlinear effects which is called a parametric interaction. When two finite amplitude sound waves having slightly different frequencies are radiated simultaneously in the medium, a sound wave having a frequency equal to the sum and difference of the two waves is produced by the nonlinear interaction (parametric interaction) of the two sound waves. Accordingly, if the original two sound waves are ultrasonic waves and the difference therebetween is so selected as to be an audio frequency, an audible sound is generated by the parametric interaction.
Assuming that the ultrasonic wave amplitude-modulated by the audio signal is radiated in the air, an ultrasonic sound field (parametric array) having a spectrum such as shown in the right-hand portion of FIG. 3 can be formed. As a result, by the parametric interaction between the carrier wave and upper and lower sideband waves, the original audio signal having the difference frequency thereof is produced in the air. The audio signal so produced reflects the directivity of the ultrasonic wave. The ultrasonic wave has a wavelength shorter than the audio frequency and is effective to provide a sound source having super directivity. Accordingly, by this method, it is possible to realize a low frequency sound source having super directivity. Moreover, the modulated ultrasonic wave radiated from the ultrasonic wave radiator is referred to as a primary wave, and an audio frequency resulting from the parametric interaction of the primary wave is referred to as a secondary wave.
However, since the parametric loudspeaker is a system utilizing the nonlinearity of a medium for producing the secondary wave, which is at the audio frequency, from the primary wave, the conversion efficiency is extremely low. By way of example, in order to obtain the secondary wave sound pressure level of about 90 dB which is a practically acceptable level, a high primary wave sound pressure of 140 dB or higher is necessitated. It is known that, when listeners are radiated by such a powerful ultrasonic wave, they will suffer from adverse effects such as, for example, hearing impairment, dizziness or headache. Accordingly, in order to put the parametric loudspeaker to practical use, it is necessary to install between the ultrasonic wave radiator 8 and a listener 9 a low bandpass acoustic filter 10 operable to intercept the primary wave, but to allow the passage of only the secondary wave as shown in FIG. 2.
What has hitherto been used as the acoustic filter consists of a so-called sound absorbing material such as fabric, felt or glass wool, which relies on its peculiar characteristic to absorb sounds of a particular band, or a cavity type muffler having a structure effective to attenuate only a particular frequency, but any one of the conventional sound absorbing material and the cavity type muffler is not suited for use as an acoustic filter for the parametric loudspeaker because the conventional sound absorbing material is manufactured with a view to attenuating only the audio frequency and because the cavity type muffler is difficult to design for an ultrasonic wave band.
In addition, in order to produce efficiently the secondary wave from the primary wave, the distance of propagation of the primary wave must be long. While the sound field in which the parametric interaction takes place is regarded as a sort of vertical array and is therefore called a parametric array, the length for which the parametric array is sufficiently completed is about 8 m at, for example, 40 kHz, although it varies with the frequency of the carrier wave, sound pressure level of the primary wave and so on. Therefore, where the acoustic filter is installed in front thereof, since the length of the parametric array (hereinafter referred to as array length) is shortened, there is a problem in that the sound pressure level of the secondary wave being reproduced is lowered along with a deterioration in directivity. Moreover, since a space for demodulation which is called the parametric array is in principle required for the production of the secondary wave, there is also a problem in that the depth of the loudspeaker tends to be lengthened and the space for installation is limited.
Yet, when the ultrasonic wave radiator 8 is secured to the ceiling of a building as shown in FIG. 4, even though the acoustic filter 10 is effective to completely intercept the ultrasonic wave, a listener 9b distant from the loudspeaker will be directly showered with the ultrasonic wave radiated from the ultrasonic wave radiator 8 and a listener 9a immediately below the acoustic filter will also be radiated with the ultrasonic wave which has been reflected from a wall or the like in the surroundings. Even though the ultrasonic wave has a super directivity, the level of the ultrasonic wave scattering in this manner within a room attains a level that cannot be considered sufficiently safe.
Furthermore, if not only is the directivity rendered super, but if also the directivity can be freely controlled should the necessity arise, advantages can be achieved. However, since the directivity of the loudspeaker, regardless of whether a direct radiator-type or a horn type, depends on the shape of the horn and the size of a vibrating plate, it has been difficult to control it freely. What has been hitherto used is a method in which the shape of the horn side wall is changed or a diffuser plate is provided. By way of example, if the angle of the movable side wall 3a which is a portion of the horn side wall is made adjustable as shown in FIG. 1, it is possible to achieve a narrow directivity when the movable side wall 3a is held at a position A, and a wide directivity when it is held at a position B. However, the range over which the directivity can be changed with this method is relatively narrow, and there is a problem in that the limit of the narrow directivity is particularly fixed by the shape of the horn side wall and the length of the horn.